1. Field of the Invention
The present invention relates to an image forming apparatus, and more specifically, to an image forming apparatus provided with an intermediate transfer belt.
2. Description of the Related Art
So far, in an image forming apparatus of the electrophotography system, image formation is carried out by entirely charging an electrophotographic photoreceptor (hereinafter often simply referred to as “photoreceptor”) by a charging unit, irradiating the charged photoreceptor with exposure light to form an electrostatic latent image, developing this electrostatic latent image with a toner, and then transferring a toner image onto a medium to be transferred such as paper. And, after optionally removing the residual toner by a cleaning unit, the transferred photoreceptor is repeatedly used for a next image forming process. In the light of the above, since an electric or mechanical external force is directly applied to the surface of an electrophotographic photoreceptor by a charging unit, a developing unit, a transfer unit, a cleaning unit, etc., the electrophotographic photoreceptor is required to have durability against such an external force.
Also, a corona discharge system for generating corona discharge to charge a photoreceptor using corotron or scorotron has hitherto been employed as a charging unit for charging the photoreceptor. However, in the case of the corona discharge system, there is encountered such a problem that the surface of the photoreceptor changes its nature due to corona products generated with the progress of corona discharge, such as ozone and NOx, whereby a phenomenon such as image blurring and deterioration of the photoreceptor takes place. Further, in the case of the corona discharge system, the charging efficiency is so poor that there is a defect of requiring a large electric power for the sake of sufficiently charging the photoreceptor.
Then, in recent years, a contact charging system for bringing a charging member into direct contact with a photoreceptor and applying an electric voltage has been put into practical use in place of the corona discharge system (for example, see JP-A-1-211779). The contact charging system is high in the charging efficiency as compared with the corona discharge system and at the same time, is remarkably low in the generation amount of ozone, etc.
However, in the case of employing such a contact charging system, since a mechanical external force due to the charging member is applied to the surface of the photoreceptor, abrasion of the photoreceptor largely increases. Also, if a local deteriorated portion is present in the photoreceptor, a local high electric field is applied to the deteriorated portion at the time of contact charging to form an electrical pinhole, thereby causing an image quality defect.
Also, the transfer system in such an image forming apparatus is broadly classified into a system for directly transferring a toner image on the surface of the photoreceptor onto a medium to be transferred and a system for primarily transferring a toner image on the surface of the photoreceptor onto an intermediate transfer body such as an intermediate transfer belt and then secondarily transferring the primarily transferred image onto a medium to be transferred (intermediate transfer system). Of these, the intermediate transfer system is broadly utilized especially in a full-color image forming apparatus because it is possible to reproduce a color image by color separating a color original image to form toner images of prescribed colors (for example, black, cyan, magenta, and yellow) and superimposing these toner imagers on the intermediate transfer belt (for example, see JP-A-9-138539).
In the case of employing such an intermediate transfer system, since the surface of the photoreceptor comes into contact with the intermediate transfer belt, damage or an increase of abrasion of the photoreceptor is likely generated, too. Also, if a foreign matter is incorporated into the image forming apparatus, there was the case where in transferring the toner image, the foreign matter sticks into the surface of the photoreceptor to generate leakage of the photoreceptor, thereby causing the generation of an image quality defect. Further, there was a problem that if the foreign matter is large, the stuck foreign matter reaches a substrate of the photoreceptor, thereby readily generating leakage of the photoreceptor.
As a method for improving these problems, for example, a method for enhancing the mechanical strength by making the surface of the electrophotographic photoreceptor hard is reviewed. According to this method, it is designed to suppress the damage or abrasion of the photoreceptor caused due to the employment of the contact charging system or intermediate transfer system. In particular, by making the surface of the photoreceptor thoroughly harder than the surface of the intermediate transfer belt, it is designed to suppress the damage or abrasion of the surface of the photoreceptor caused due to the contact with the surface of the intermediate transfer belt.
However, in the case where the surface of the photoreceptor is made hard while making the surface of the intermediate transfer belt relatively soft, there are generated such inconveniences that in transferring the toner image, the intermediate transfer belt is damaged and that an image quality defect such as out of color registration is generated. Thus, even if the life as the photoreceptor is prolonged, not only the life of the intermediate transfer belt becomes short, but also an image quality defect is likely generated. For this reason, it is difficult to stably form an image having a good image quality over a long period of time as the whole of the image forming apparatus.
Then, a method for not only making the surface of the electrophotographic photoreceptor sufficiently hard but also making the surface of the intermediate transfer belt sufficient hard is reviewed.
However, in this case, since the hard materials come into contact with each other in transferring the toner image, there is encountered such a problem that damage or abrasion is likely generated in both the photoreceptor and the intermediate transfer belt. Further, if the mechanical strength of the photoreceptor and the intermediate transfer belt is high, in removing the residual toners on the surface of the photoreceptor and the intermediate transfer belt by a cleaning unit such as a cleaning blade, there is encountered such a problem that the cleaning blade is likely damaged, the life of the cleaning blade becomes short, and a fragment of the damaged cleaning blade becomes a foreign matter which then sticks into the surface of the photoreceptor, thereby likely generating leakage of the photoreceptor. For this reason, it is difficult to stably form an image having a good image quality over a long period of time as the whole of the image forming apparatus.